The present invention relates to optical encoders and, more specifically, to an optical encoder capable of detecting two-dimensional displacement.
Various types of encoders are presently used in measuring instruments and machine tools and, particularly in recent years, in information equipment, to detect relative displacement between two members. In particular, optical encoders are commonly used in places where non-contact-type displacement detection is required.
The optical encoders have a structure in which a main scale and an index scale each having a grating are provided on respective members which make relative movement. For instance, the main scale is irradiated with light passed through the grating of the index scale, and the light reflected or transmitted from the grating of the main scale is detected by a photodetector. A relative displacement between the two members are determined based on a phase variation and other parameters of the detection signal.
A three-grating-type, one-dimensional optical encoder is disclosed in U.S. Pat. No. 5,026,164.
However, conventional general optical encoders are only capable of one-dimensional measurement of linear or rotational displacement. That is, a single encoder cannot detect two-dimensional displacement between two members.